Beneath the continent of Africa and the Pacific ocean are two massive masses located in the lowest part of the Earth’s mantle and surrounding the Earth’s core, occupying about three to nine percent of the Earth’s volume.
Of course, there is no way to see the Earth’s core directly. The deepest hole ever excavated, which is mildly amusingly called the “entry to hell,” has reached 12,263 meters and still has to go a lot further to break through the earth’s crust and reach the layers below. Still, we are able to study below the surface quite effectively by taking advantage of earthquakes in a technique known as seismic tomography.
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When earthquakes occur, energy waves radiate in all directions. By measuring tremors from various locations on the surface, scientists can create a map of the Earth’s interior. Because rocks and fluids on Earth have different densities, waves travel through these masses at different speeds, allowing geologists to understand what kind of material the waves are traveling through.
When this technique was just beginning to be used, two large and strange structures called large low shear velocity regions (LLSVPs) were found. In these areas, often called “amorphous masses,” waves move more slowly than the surrounding lower mantle. Beneath Africa, the region known as the “Tuzo” is thought to be about 800 kilometers high, or about 90 the size of Mount Everest.
Unfortunately, while we have a few solid ideas of what these masses are, we’re still not entirely sure. The objects are assumed to be made of a different material, given that they are denser than the surrounding mantle, but it is not possible to tell exactly what it is or its exact density based on seismic tomography data alone.
One notable hypothesis is that LLSVPs are heaps of oceanic crust that have sunk and accumulated over billions of years. Another theory, which is a little more amusing, proposes that these fragments are fragments of an ancient planet.
Could it be Theia?
Theia was a hypothetical Mars-sized planet that crashed into Earth about 4.5 billion years ago, causing enough rock to be ejected into space to form the Moon. It is suggested that these masses are actually fragments of Theia itself, the denser mantle of the proto-planet that merged with that of Earth during the collision. In 2021, a team modeled simulations of the scenario and found that Theia’s mantle could only survive if it had between 1.5 and 3.5 percent higher density.
While we don’t know for sure what the masses are, and we won’t see them directly, at least until the distant future, our techniques for probing the ground are constantly evolving.
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